Objective To explore variations of the serum cardiac troponin T (cTNT) level for patients who underwent radiotherapy after mastectomy and to investigate the relationship between cTNT and myocardial damage. Methods Patients undergoing radiotherapy after mastectomy were selected and divided into the L group (50 patients underwent left mastectomy) and R group (50 patients underwent right mastectomy). The control group included 50 patients underwent radiotherapy at locations other than the thorax. The cTnT level of all patients was detected by the colloidal gold method before, in the middle of, and at the end of radiotherapy, 3 months, 6 months, and 12 months after radiotherapy, respectively. cTnT＞0.03 ng/mL was the criterion for the diagnosis of myocardial damage. Results Positive rates of the serum cTNT level of the group L and group R after radiotherapy were significantly different to that of the control group (P<0.05), while the difference of the L group and R group was not statistically significant (P>0.05). Compared to the serum cTNT level of the L group and R group detected before radiotherapy, the serum cTNT level detected at the end of radiotherapy increased significantly and reached the peak value. The differences were statistically significant (P<0.01). The serum cTNT level was back to normal 6 months after radiotherapy. Conclusion Postmastectomy radiotherapy (PMRT) may lead to the significant rise of serum cTNT level and increase the incidence of myocardial damage. Variations of the cTNT level can predict radioactive myocardial damage at early stage and is helpful for the early prevention and treatment of complications and sequelae relevant to the radiotherapy.